An ice cube is floating in water above which a layer of a lighter oil is poured. As the ice melts completely, the level of water surface (or interface) and upper level of oil will

When the oil is poured, the fraction of ice in the water decreases, i.e., volume of ice melted into water is greater than volume of water displaced by ice. So water level rises. Overall volume of ice will decrease as it melts. So the upper level of oil falls.